P-39 Airacobra

[KJ_Lesnick]

From what I remember they had trouble fitting the turbocharger to the design and making everything work: Would an interim arrangement including something like one of the following

• Twin-stage, single-speed supercharger
  
• Single-stage, twin-speed supercharger
  

Have helped for interim purposes until they could either put a supercharger, provided that could be done?

[maxmwill]

Well, if memory serves me(and you know those vicious but capricious Time Lords), the airframe may have been too small for it, or that Bell was unwilling to design the aircraft for a super charged Allison, or did not count on a lot of fighting taking place above the ceiling the Cobra was designed for, and later on the Cobra wasn't considered for further development as the Mustang got(remember how dismal the earlier models of the Mustang were, especially the British experience? Although the Army did put it to good use as the Apache).

The P47(which model was developed from another design that was not to be the '47), being a much larger airframe did have a supercharger.

[jcf]

The Allison in the P-39 was supercharged, the only V-1710 version that didn't
have an engine-mounted supercharger was the model built as an airship engine.

The turbocharger installation on the XP-39 functioned as a second stage supercharger
and the installation was not ideal:
it was mounted directly beneath the engine, exhaust flow path was not optimal and the
intercooler was too small due to airframe envelope restrictions. The related scoops and
duct work also created excessive drag. When the NACA cleaned up the airframe, drag
was reduced markedly and potential speed increased.

The ugly reality was that the airframe cleanups didn't balance the inevitable weight gain of
turning a prototype into an operational aircraft, and retaining the turbo-charger would have
just complicated the issue without any real improvement.

As to why GM waited so long to develop multi-speed, multi-stage supercharger systems for
the V-1710 and then when they finally did pursue concepts did it in such a seemingly haphazard
fashion, well the standard answer is that GM management didn't want to spend the money to
give Allison the necessary resources.

Could the V-1710 have been redesigned to incorporate a Merlin style engine-mounted
single-stage/twin-speed supercharger?, and then later a two-stage/two-speed system?
Sure, but it would have required extensive redesign and new tooling to produce the parts
and production delays to incorporate the new design on the line, all of which spelled co$t
to the GM board. Thus the separate auxiliary supercharger designs that eventually were
produced and used in the P-63 and other types.

Much more discussion on ideas for the Bell on the existing thread for the P-39 and P-63:
http://www.whatifmodelers.com/index.php/topic,10288.0/highlight,p-39.html

[KJ_Lesnick]

The Allison in the P-39 was supercharged, the only V-1710 version that didn't have an engine-mounted supercharger was the model built as an airship engine.

That I know, however those were single-stage, single-speed systems:  What I described would have jacked up the critical altitude by either providing two speeds, or adding an extra stage.

The turbocharger installation on the XP-39 functioned as a second stage supercharger and the installation was not ideal: it was mounted directly beneath the engine, exhaust flow path was not optimal and the intercooler was too small due to airframe envelope restrictions.

1. I assume the turbocharger was placed where it was to minimize volume?

2. As for the airflow not being optimal: Could that have been rectified in the confines of the airplane?

3. When you say airframe envelope restrictions, do you mean like some speed and altitude limit?

The related scoops and duct work also created excessive drag. When the NACA cleaned up the airframe, drag was reduced markedly and potential speed increased.

I do know they made modifications, but I'm curious of the exact details if you know

As to why GM waited so long to develop multi-speed, multi-stage supercharger systems for the V-1710 and then when they finally did pursue concepts did it in such a seemingly haphazard fashion, well the standard answer is that GM management didn't want to spend the money to give Allison the necessary resources.

I always thought it was that the USAAC generally preferred turbochargers to complex supercharger arrangements: The US Navy seemed almost the opposite with the F4F being one of the world's (if not *the* world's first aircraft) with a twin-stage, twin-speed supercharger (not sure if it had inter/after-cooling).  

1. Was there anything that could have (as my dad elegantly says) put a boot in their donkey and got them underway?
2. When did people in the US realize that you could actually shape the exhaust-stubs to produce thrust?

[jcf]

No, physical envelope, not performance envelope. It was simply too small.

[KJ_Lesnick]

joncarrfarrelly

No, physical envelope, not performance envelope. It was simply too small.

Sorry about that, I didn't understand what you said: Would a stretch of two feet have worked?

I'm thinking about two things

1. Was it possible to build a turbocharger at a later date that would have fit in the design?  Technology often allows for miniaturizaiton and stuff

2A. The USN pursued a twin-stage, twin-speed supercharger arrangement for it's F4F-3: Was it in the prototype from the outset, or added merely before service entry?
2B. Was there anything the USAAF could have done to put a figurative boot in the donkey of GM to pursue a more elaborate supercharger to quickly compensate at least for a short-term mod?  At least it'd be better than a single-stage, single-speed layout.

[pyro-manic]

See the P-63.

[KJ_Lesnick]

See the P-63.

How does a hydraulic turbosupercharger work?  I thought a turbocharger used exhaust to drive a turbine, which then in turn drives an impeller?

[wuzak]

See the P-63.

How does a hydraulic turbosupercharger work?  I thought a turbocharger used exhaust to drive a turbine, which then in turn drives an impeller?

The auxiliary stage supercharger was driven, in Allisons, by a variable speed transmission (fluid coupling) which mimicked the turbocharger somewhat.

Apart from a few 2 stage engines which had the carburettor on the auxiliary stage, and a few which had the intercooler, the core engine of the 2 stage Allison was the same as used in turbo applications. They simply bolted on the extension drive, supercharger, etc.

[wuzak]

While they were owned by GM, Allison were still quite small and had a small engineering team. They were developing all sorts of things for the V-1710 - including extension shafts and remote gearboxes for aircraft like the P-39. They were also devloping the basic design and the related V-3420.

So 2 speed and 2 stage options were in the future. Plus they didn't spend money on things they didn't have orders for.

[wuzak]

joncarrfarrelly

No, physical envelope, not performance envelope. It was simply too small.

Sorry about that, I didn't understand what you said: Would a stretch of two feet have worked?

That's what they did for the P-39E and P-63.

[KJ_Lesnick]

wuzak

The auxiliary stage supercharger was driven, in Allisons, by a variable speed transmission (fluid coupling) which mimicked the turbocharger somewhat.

Okay, so it was a mechanical staged supercharger, with an auxiliary-stage variable speed fluid-clutched supercharger?

While they were owned by GM, Allison were still quite small and had a small engineering team. . . Plus they didn't spend money on things they didn't have orders for.

Had they been given an order to produce it, could they have by the time the P-39 came online with the resources available?

That's what they did for the P-39E and P-63.

That's why I brought it up...